Systems and methods employing a plurality of signal amplitudes to record the presence of multiple objects

ABSTRACT

A system and method for identifying objects. A preferred embodiment employs a radio transceiver on each object. Each article is identified by recording an amplitude at which the respective transceiver exhibits a nonlinearity.

This Application is a continuation of application Ser. No. 08/645,492 ofJEROME D. JACKSON filed May 13, 1996 now U.S. Pat. No. 5,864,301. Thecontents of application Ser. No. 08/645,492 filed May 13, 1996 is hereinincorporated by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates generally to a system and method for identifyingobjects and, more particularly, to a system for identifying objectshaving transceiver tags.

2. Description of Related Art

Automatic identification systems employing radio sensitive tags havebeen proposed for tracking of people, animals, vehicles and baggage. Forexample, U.S. Pat. No. 5,204,681, issued Apr. 20, 1993 to Greene,describes a system having a target affixed to an object to beidentified, a transmitter for generating interrogation signals, and areceiver having a signal processor for detecting a target. Each targetincludes multiple resonators resonant at respective frequencies. Theresonant frequencies associated with a particular target are a subset ofthe frequencies detectable by the receiver, and provide the target withidentification data.

A problem with the system described in U.S. Pat. No. 5,204,681 is thatthe system may not be able to identify a target in the presence of othertargets. When more than one target is present, the signal processor maybe unable to correlate the combination of detected resonant frequencieswith any particular target, because the detected combination will notcorrespond to any one target, but will instead correspond to thecombined set of resonant frequencies from all of the targets.

This problem may be addressed to some extent with a detection amplitudethreshold for each resonant frequency. With this thresholding scheme,the signal processor does not consider a resonant frequency to bepresent unless the received amplitude is over the threshold. A problemwith this scheme is that, in order to identify each object, the movementof the objects relative to the transmitter and receiver must be highlyregimented such that at any one time only one target is transmittingsignals to the receiver above the threshold.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide a system and methodfor identifying an object in the presence of other objects.

According to an aspect of the present invention, in a system including aplurality of articles each having a respective circuit for receiving aninterrogation signal and transmitting a respective circuit signal, thecircuit signal being a function of the interrogation signal, and havinga nonlinearity, the nonlinearity corresponding to a respective amplitudelevel of the interrogation signal, a method comprises detecting, foreach circuit, the respective amplitude level by varying an amplitudeused to transmit the interrogation signal to the plurality of articles;and registering the presence of each article, in response to theprevious step.

According to another aspect of the present invention, in a systemincluding a plurality of articles each having a respective circuit forreceiving an interrogation signal and transmitting a respective circuitsignal, the circuit signal being a function of the interrogation signal,and having a nonlinearity, the nonlinearity corresponding to arespective amplitude level of the interrogation signal, a methodcomprises detecting, for each circuit, the respective amplitude level byvarying an amplitude used to transmit the interrogation signal to theplurality of articles; and recording the presence of each article, inresponse to the previous step.

According to another aspect of the present invention, there is apparatusfor a system including a plurality of articles each having a respectivecircuit for receiving an interrogation signal and transmitting arespective circuit signal, the circuit signal being a function of theinterrogation signal, and having a nonlinearity, the nonlinearitycorresponding to a respective amplitude level of the interrogationsignal. The apparatus comprises a detector that detects, for eachcircuit, the respective amplitude level by varying an amplitude used totransmit the interrogation signal to the plurality of articles, andperforming a comparison; and a recorder that records the presence ofeach article, in response to the comparison.

According to another aspect of the present invention, in a systemincluding a plurality of articles each having a respective circuit forreceiving an interrogation signal and transmitting a respective circuitsignal, the circuit signal being a function of the interrogation signal,and having a nonlinearity, the nonlinearity corresponding to arespective amplitude level of the interrogation signal, an apparatuscomprises means for detecting, for each circuit, the respectiveamplitude level by varying an amplitude used to transmit theinterrogation signal to the plurality of articles, and performing acomparison; and means for recording the presence of each article, inresponse to the comparison.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram of an article identification system in accordancewith a first preferred embodiment of the present invention.

FIG. 2 is a block diagram of the transmitter shown in FIG. 1.

FIG. 3 is a block diagram of the receiver shown in FIG. 1.

FIG. 4 is a block diagram of an identification label in the firstpreferred system.

FIG. 5 is a diagram of one of the circuits shown in FIG. 4.

FIG. 6 is a diagram of a device in the circuit shown in FIG. 5.

FIG. 7 is a curve of the current-voltage characteristics of the deviceshown in FIG. 6.

FIG. 8A. is a curve of the frequency response of one of the circuitsshown in FIG. 4.

FIG. 8B is a curve of the frequency response of th e label shown in FIG.4.

FIG. 9A-9E are power curves generated by the first preferred system atdifferent frequencies.

FIG. 10. is a flow chart of a processing performed by the firstpreferred system.

FIG. 11 is a flow chart of a part of the processing shown in FIG. 6.

FIG. 12 is a diagram of an article identification system in accordancewith a second preferred embodiment of the present invention.

FIG. 13 block diagram of an identification label in the second preferredsystem.

FIG. 14 is a curve of the frequency response of the label shown in FIG.13.

FIG. 15 block diagram of another identification label in the secondpreferred system.

FIG. 16 is a curve of the frequency response of the label shown in FIG.15.

FIG. 17A-17E are power curves generated by the second preferred systemat different frequencies.

FIG. 18 is a flow chart of a processing performed by the secondpreferred system.

FIG. 19 is a flow chart of a part of the processing shown in FIG. 17.

FIG. 20 is a flow chart of another portion of the processing shown inFIG. 17.

FIG. 21 is a diagram of an identification label circuit in accordancewith an alternative embodiment of the present invention.

The accompanying drawings which are incorporated in and which constitutea part of this specification, illustrate embodiments of the inventionand, together with the description, explain the principles of theinvention, and additional advantages thereof. Throughout the drawings,corresponding elements are labeled with corresponding reference numbers.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 shows an article identification system 1000 in according to afirst preferred embodiment of the present invention. System 1000includes a conveyor belt 1010 for moving suitcase 1020 in the directionof arrow 1012. Label 2020 is attached to suitcase 1020. Transmitter 1115and antenna 1110 transmit interrogations signals to label 2020. Receiver1125 and antenna 1120 receive response signals from label 2020.Processor 1200 controls transmitter 1115 and receiver 1125 by sendingcommands signals over signal bus 1225. Processor 1200 executes program1215 stored in memory 1210.

FIG. 2 shows a block diagram of transmitter 1115. Tunable sine wavegenerator 1116 receives a frequency select command from processor 1200,through bus interface 1119 and signal line 1146, and sends a sinusoidsignal of the selected frequency to variable power amplifier 1118 viasignal line 1149. Amplifier 1118 receives a power select command fromprocessor 1200 through buss interface 1119 and signal line 1148.Amplifier 1118 amplifies the sinusoid signal and sends the amplifiedsignal to antenna 1110 via signal line 1151, causing antenna 1110 toradiate the amplified signal at the selected power.

FIG. 3 shows a block diagram of receiver 1125 shown in FIG. 1. Tunableband pass filter 1128 receives a band select command from processor1200, through bus interface 1129 and signal line 1168, and filters outsignals received from antenna 1120 that are outside of the selectedband. Filter 1128 passes the filtered signal to demodulator 1127 viasignal line 1169. Demodulator 1127 is an amplitude modulation detector.Demodulator 1127 passes the demodulated signal via signal line 1171 toA/D converter 1126, which converts the level received from demodulator1127 into a digital number, and sends the digital number to processor1200 through signal line 1173 and bus interface 1129.

The time constant of demodulator 1127 should be relatively long so thatthe output of demodulator 1127 will have little ripple, allowingprocessor 1200 to discriminate between small changes in signal level.Although this long time constant makes it difficult to detect rapidchanges in signal level, the preferred system does not require suchrapid detection, as will be apparent from the description below.

FIG. 4 shows label 2020 attached to suitcase 1020. Label 2020 includesresonant circuits 2111, 2100, 2101, and 2103. Each resonant circuit isconfigured to respond to a certain received frequency by transmitting atanother frequency. Circuits 2111, 2100, 2101, and 2103 are each lessthan one square inch, and have a uniform orientation. The distancebetween adjacent resonant circuits is less than one inch. Because ofthis uniform orientation and small inter-circuit, intra-label,separation distances, and because the distance between the label 2020and transmitting antenna 1110 is at least several feet, each of theresonant circuits within label 2020 has a substantially commonorientation relative to antenna 1110. Similarly, because the distancebetween label 2020 and receiving antenna 1120 is at least several feet,each of the resonant circuits within label 2020 has a substantiallycommon orientation relative to antenna 1120.

FIG. 5 shows resonant circuit 2111. Inductor 3030 and capacitor 3035constitute a tank circuit functioning as the receiver 3033 of circuit3000, with inductor 3030 functioning as the antenna of the receiver.Transistor 3005, diode 3010, capacitor 3020, resistor 3025, andcapacitor 3015 function as a power supply, with capacitor 3020functioning as an AC bypass and resistor 3025 functioning to biastransistor 3005. The receiver and power supply of circuit 2111 aredescribed in more detail in U.S. Pat. No. 3,859,652, issued Jan. 7, 1975to Hall et al., the contents of which is herein incorporated byreference.

Inductor 3040, capacitor 3045, and voltage limiter 3100 function as thetransmitter of circuit 2111, with inductor 3040 functioning as theantenna of the transmitter. Circuit 2111 responds to a signal having afrequency Tref, radiated by antenna 1110, by transmitting a signalhaving a frequency Rref. It is presently preferred that capacitor 3045be a thin film capacitor having a capacitance substantially independentfrom the voltage across capacitor 3045.

FIG. 6 shows voltage limiter 3100 of the circuit of FIG. 5. Zener diode3105 and 3110 are each diffused PN junction devices having heavy dopingon the substrate side of the junction, and having a tunnel breakdownvoltage of several volts. The anode of zener diode 3105 is coupled tothe anode of zener diode 3110. The cathode of zener diode 3105constitutes the first terminal of voltage limiter 3100, and the cathodeof zener diode 3110 constitutes the second terminal of voltage limiter3100. Zener diodes 3105 Osand 3110 limit the intensity of the signaltransmitted by circuit 2111 at frequency Rref.

The natural frequency of transmitter 3043 is essentially determined byinductor 3040 capacitor 3045, and parasitic capacitances in circuit2111. For example, to achieve a transmitter resonance of approximately0.93 Megahertz (Rref=0.93 Megahertz), inductor 3040 may be 5,800μμhenries and capacitor 3045 may be 5 microfarads.

Similarly, the natural frequency of receiver 3033 is essentiallydetermined by inductor 3030 and capacitor 3035. In other words, thereceiver resonance of circuit 2111 (Tref) is controlled by inductor 3030and capacitor 3035.

FIG. 7 shows the current-voltage characteristic of voltage limiter 3100.As shown in FIG. 7, circuit 3100 sinks a substantial amount of currentwhen the voltage across the first and second terminals exceeds V1 or-V2. V1 is a function of the zener breakdown voltage of diode 3110 plusthe forward bias current drop of diode 3105. Similarly, V2 is a functionof the zener breakdown voltage of diode 3105 plus the forward biascurrent drop of diode 3110.

FIGS. 8A and 8B are frequency response curves. (The curves shown inFIGS. 8A and 8B do not directly represent processing performed byprocessor 1200 and program 1215 but are included in this descriptionbecause they illustrate a relevant characteristic of the circuits inlabel 2020). FIG. 8A represents a retransmission response of circuit2111, which transmits at a frequency Rref. As shown in FIG. 8A, whentransmitter 1110 transmits at a frequency Tref, the intensity of thesignal transmitted by circuit 2111 is at a maximum.

Circuits 2103, 2101, and 2100 have a structure similar to that ofcircuit 2111 described above, except that each resonant circuit hascomponent values corresponding to its respective resonant frequency.

FIG. 8B represents a composite retransmission response of label 2020.This composite response is determined by the combination of circuits2111, 2100, 2101, 2103. Circuit 2111 transmits at a frequency Rref, andtransmits at a maximum intensity when it receives a signal Treftransmitted by antenna 1110. Circuit 2103 transmits at a frequencyRbit3, and transmits at a maximum intensity when it receives a frequencyTbit3 transmitted by antenna 1110. Circuit 2101 transmits at a frequencyRbit1, and transmits at a maximum intensity when it receives a frequencyTbit1 transmitted by antenna 1110. Circuit 2100 transmits at a frequencyRbit0, and transmits at a maximum intensity when it receives a frequencyTbit0 transmitted by antenna 1110. In other words, the natural frequencyof transmitter 3043 in circuit 2111 is Rref, the natural frequency oftransmitter 3043 in circuit 2103 is Rbit3, the natural frequency oftransmitter 3043 in circuit 2101 is Rbit1, and the natural frequency oftransmitter 3043 in circuit 2100 is Rbit0. The natural frequency ofreceiver 3033 in circuit 2111 is Tref, the natural frequency of receiver3033 in circuit 2103 is Tbit3, the natural frequency of receiver 3033 incircuit 2101 is Tbit1, and the natural frequency of receiver 3033 incircuit 2100 is Tbit0.

Label 2020 represents a number having 4 bit positions, each positioncorresponding to a respective frequency. Label 2020 represents thenumber 1011 because label 2020 has circuits corresponding to bitpositions 0, 1, and 3, and has no circuit corresponding to bit position2. More specifically, circuit 2100 corresponds to bit position 0 becausecircuit 2100 has a maximum response at transmitted frequency Tbit0.Circuit 2101 corresponds to bit position 1 because circuit 2101 has amaximum response at Tbit1. Circuit 2103 corresponds to bit position 3because circuit 2103 has a maximum response at transmitted frequencyTbit3.

Processor 1200 detects the value of a particular bit by varying thepower transmitted by transmitter 1115 at the frequency corresponding tothe particular bit, e.g. frequency Tbit0, to detect whether a breakpointis present. The presence of a breakpoint means the corresponding bit is1 and the absence of a breakpoint means the corresponding bit is 0.Processor 1200 follows this procedure for each bit position, to detectthe value of the label. In other words, processor 1200 detects thenumber 1011 by sequentially transmitting on each frequency (Tbit3,Thit2, Tbit1 and Tbit0) and detecting an intensity (average power) atantenna 1120, as described in more detail below.

Each label includes a reference circuit having a maximum response atTref, regardless of the value of the label.

FIG. 9A shows a curve REF₁₃ 2020 of signal intensity (average power) online 1171 at the output of demodulator 1127, versus signal intensity(average power) transmitted by transmitter 1115, when band pass filter1128 is set to a band having a center frequency at Rref and sine wavegenerator 1116 is set to a frequency Tref. The curve REF₋₋ 2020 resultsfrom the response of circuit 2111. Between transmitted intensities I1and I2, the signal on line 1171 is an increasing function of intensitytransmitted by transmitter 1115 until the transmitting intensity reachesa breakpoint at I2, at which point the response of circuit 2111flattens. Voltage limiter 3100 in circuit 2111 causes this flattenedresponse by limiting the voltage in transmitter 3043 in circuit 2111,and by detuning transmitter 3043 as the increasing voltage intransmitter 3043 changes the capacitance of the PN junctions in voltagelimiter 3100.

Label 2020 may be conceptualized as a circuit composed of multiplecircuits (circuits 2111, 2103, 2101, and 2100). Label 2020 receives afirst signal (from antenna 1110) and transmits a second signal, thesecond signal being a function of the first signal, the function havinga nonlinearity at a first transmission amplitude (I2) corresponding to afirst frequency (Tref) of the first signal.

FIG. 9B shows a curve BIT0₁₃ 2020 of signal intensity on line 1171 atthe output of demodulator 1127, versus signal intensity transmitted bytransmitter 1115, when band pass filter 1128 is set to a band having acenter frequency at Rbit0, and sine wave generator 1116 is set to afrequency Tbit0. The curve BIT0₋₋ 2020 results from the response ofcircuit 2100. Between transmitted intensities I1 and I2, the signal online 1171 is an increasing function of intensity transmitted bytransmitter 1115 until the transmitting intensity reaches a breakpointat I2, at which point the response of circuit 2100 flattens. Voltagelimiter 3100 in circuit 2100 causes this flattened response by limitingthe voltage in transmitter 3043 in circuit 2100, and by detuningtransmitter 3043 as the increasing voltage in transmitter 3043 changesthe capacitance of the PN junctions in voltage limiter 3100.

FIG. 9C shows a curve BIT1₋₋ 2020 of signal intensity on line 1171 atthe output of demodulator 1127, versus signal intensity transmitted bytransmitter 1115, when band pass filter 1128 is set to a band having acenter frequency at Rbit1, and sine wave generator 1116 is set to afrequency Tbit1. The BIT1₋₋ 2020 results from the response of circuit2101. Between transmitted intensities I1 and I2, the signal on line 1171is an increasing function of intensity transmitted by transmitter 1115until the transmitting intensity reaches a breakpoint at I2, at whichpoint the response of circuit 2101 flattens. Voltage limiter 3100 incircuit 2101 causes this flattened response by limiting the voltage intransmitter 3043, and by detuning transmitter 3043 as the increasingvoltage in transmitter 3042 changes the capacitance of the PN junctionsin voltage limiter 3100.

FIG. 9D shows a curve of signal intensity on line 1171 at the output ofdemodulator 1127, versus signal intensity transmitted by transmitter1115, when band pass filter 1128 is set to a band having a centerfrequency at Rbit2, and sine wave generator 1116 is set to a frequencyTbit2. The curve shown in FIG. 9D has no breakpoint because label 2020has no circuit corresponding to bit 2. In other words, the curve shownin FIG. 9D has no breakpoint because there is no resonant circuitcorresponding to transmitted frequency Tbit2 and received frequencyRbit2, in proximity to antenna 1110.

FIG. 9E shows a curve BIT3₋₋ 2020 of signal intensity on line 1171 atthe output of demodulator 1127, versus signal intensity transmitted bytransmitter 1115, when band pass filter 1128 is set to a band having acenter frequency at Rbit3, and sine wave generator 1116 is set to afrequency Tbit3. The curve BIT3₋₋ 2020 results from the response ofcircuit 2103. Between transmitted intensities I1 and I2, the signal online 1171 is an increasing function of intensity transmitted bytransmitter 1115 until the transmitting intensity reaches a breakpointat I2, at which point the response of circuit 2111 flattens. Voltagelimiter 3100 in circuit 2103 causes this flattened response by limitingthe voltage in transmitter 3043 in circuit 2103, and by detuningtransmitter 3043 as the increasing voltage in transmitter 3043 changesthe capacitance of the PN junctions in voltage limiter 3100.

FIG. 10 shows a procedure, performed by processor 1200 and program 1215,for reading label 2020 on suitcase 1020. First, processor 1200 detects abreakpoint at the reference frequency, by varying the intensity of thesignal transmitted on antenna 1110, and saves the breakpoint in avariable B₋₋ REF. (step 10010). Next, Processor 1200 sets the system todetect the least significant bit of label 2020, by setting sine wavegenerator 1116 to the frequency TbitO and setting band pass filter 1128to a band centered around Rbit0 (step 10020). In step 10020, processor1200 also sets a variable L₋₋ VALUE to 0000. Processor 1200 thensearches for a breakpoint at the presently selected bit frequency, byvarying the intensity of the signal transmitted on antenna 1110 (step10030). If a breakpoint exists at the presently selected frequency, step10030 sets a variable B₋₋ PRESENT to the intensity of the breakpoint. Ifthere is a breakpoint at the present bit frequency (step 10040),processor 1200 determines whether the breakpoint at the present bitfrequency is within a certain range of the breakpoint at the referencefrequency (step 10050). In other words, step 10050 performs thecomparison:

    ABS(B.sub.13 PRESENT--B.sub.-- REF) <=TOLERANCE,

where ABS is the absolute value function, B₋₋ PRESENT is the breakpointfound in step 10030, and TOLERANCE is a constant. If the breakpoint atthe present bit is within this range, processor 1200 sets the presentbit in L₋₋ VALUE (step 10060).

As described above, step 10050 disregards any breakpoints that are notin proximity to the breakpoint of the reference circuit in label 2020.These disregarded breakpoints may correspond to circuits in other labels(labels other than label 2020). Thus, in the first preferred system,signals from other labels are rejected as noise.

Steps 10070 and 10080 repeat steps 10030, 10040, 10050, and 10060 foreach bit position and corresponding frequency. More specifically,processor 1200 executes step 10030-10060 a first time, at which timestep 10060 changes L₋₋ VALUE from 0000 to 0001. Processor 1200 thenselects the next bit frequency by setting sine wave generator 1116 tofrequency Tbit1 and setting band pass filter 1128 to a band having acenter frequency at Rbit1 (step10080). Processor 1200 then reexecutessteps 10030-10060, at which time step 60 changes L₋₋ VALUE from 0001 to0011.

Subsequently, processor 1200 selects the next bit frequency by settingsine wave generator 1116 to frequency Tbit2 and setting band pass filter1128 to a band having a center frequency at Rbit2 (step 10080). Becauseno breakpoint exists for the second bit, processor 1200 does not executesteps 10050 and 10060, and L₋₋ VALUE does not change.

Subsequently, processor 1200 selects the next bit by setting sine wavegenerator 1116 to a frequency Tbit3 and setting band pass filter 1128 toa band having a center at Rbit3 (step 10080), and reexecutes steps10030-10060, at which time L₋₋ VALUE changes from 0011 to 1011.

Thus, processor 1200 determines a set of other frequencies (Tbit0, Tbit,and Tbit3) of the first signal at which the second signal has anonlinearity at a first signal amplitude corresponding to the firsttransmission amplitude (an amplitude within tolerance of I2).

At the end of the procedure shown in FIG. 10, L₋₋ VALUE will equal 1011.Thus, L₋₋ VALUE stores an article identification signal corresponding tosuitcase 1020.

FIG. 11 shows a subprocedure of step 10010 and 10030 of FIG. 10. Theprocedure of FIG. 11 collects data points along a response curve, suchas the curve shown in FIG. 9A, by incrementally increasing thetransmitted signal intensity. To determine where a breakpoint exists,processor 1200 processes the curve by segments, and detects whether thedifference in slope of any two adjacent segments is greater than acertain threshold. More specifically, processor 1200 causes amplifier1118 to transmit an initial intensity (I1) on antenna 1110, and detectsa received intensity (through antenna 1120, filter 1128, demodulator1127, and A/D converter 1126) by squaring the value on signal line 1173at the output of A/D converter 1126 (step 11010). Processor 1200 thencauses amplifier 1118 to transmit at a second intensity, higher than theinitial intensity, and detects a second received intensity by squaringthe value on signal line 1173 (step 11020). Processor 1200 then causesamplifier 1118 to transmit at the next higher intensity, and detectsanother received intensity (step 11030). Processor 1200 then compares adifference between the slope of the current segment and the slope of theprevious segment, by comparing the absolute value of the quantity##EQU1## to 0.2 (11040). If this difference in slope is greater than0.2, a break point exists and the procedure of FIG. 11 terminates. Ifthis difference in slope in not greater than 0.2, it is determinedwhether the transmission intensity limit has been reached (step 11050).If the limit has not been reached, processor 1200 repeats steps 11030and 11040. If the limit has been reached, no breakpoint exists, and theprocedure of FIG. 11 terminates. ##EQU2## where TRANSMITTED_(T) is theintensity transmitted by amplifier 1118 in step 11030 and RECEIVED_(T)is the intensity detected by squaring the output of A/D converter 1126in step 11030. TRANSMITTED_(T-1) is the intensity transmitted in thetransmit and detect step previous to step 11030 and RECEIVED_(T-) is theintensity detected in the transmit and detect step previous to step11030. This previous transmit and detect step will be step 11020, thefirst time through the loop, or a previous invocation of step 11030,subsequent times through the loop. ##EQU3## where TRANSMITTED _(T) isthe intensity transmitted in step 11010, RECEIVED₁ is the intensityreceived in step 11010, TRANSMITTED₂ is the intensity transmitted instep 11020, and RECEIVED₂ is the intensity received in step 11020.

FIG. 12 shows an article identification system 8000 in accordance with asecond preferred embodiment of the present invention. System 8000includes a conveyor belt 1010 for moving suitcases 1020, 1025, and 1030in the direction of arrow 1012. Label 2020 is attached to suitcase 1020,label 2025 is attached to suitcase 1025, and label 2030 is attached tosuitcase 1030. Transmitter 1115 and antenna 1110 transmit interrogationsignals to labels 2020, 2025, and 2030. Receiver 1125 and antenna 1120receive response signals from labels 2020, 2025, and 2030. Processor1200 controls transmitter 1115 and receiver 1125 by sending commandssignals over signal bus 1225. Processor 1200 executes program 1216stored in memory 1210.

In the Figures describing the second preferred system, elementscorresponding to elements in the first preferred system are designatedwith corresponding reference numbers.

Labels 2025 and 2030 each have a structure similar to label 2020,including multiple resonant circuits. The labels differ from each other,however, in the combination of resonant frequencies associated with aparticular label. While label 2020 has resonant frequenciescorresponding to the respective frequencies of circuits 2111, 2100,2101, and 2103, label 2025 has a different set of resonant circuits andtherefore a different set of corresponding resonant frequencies.Similarly, label 2030 has its own set of resonant frequencies.

FIG. 13 shows label 2025 attached to suitcase 1025. Label 2025 includesresonant circuits 2111, 2101, and 2103. Label 2025 is flat so that eachof the resonant circuits has a substantially common orientation relativeto antenna 1110, and each of the resonant circuits has a substantiallycommon orientation relative to antenna 1120.

FIG. 14 represents a composite retransmission response of label 2025.This composite response is determined by the combination of circuits2111, 2101, and 2103. Label 2025 represents a number having 4 bitpositions, each position corresponding to a respective frequency. Label2025 represent the number 1010 because label 2025 has circuitscorresponding to bit positions 1 and 3, and has no circuitscorresponding to bit positions 0 and 2.

FIG. 15 shows label 2030 attached to suitcase 1030. Label 2030 includesresonant circuits 2111, 2101, and 2100. Label 2030 is flat so that eachof the resonant circuits has a substantially common orientation relativeto antenna 1110, and each of the resonant circuits has a substantialcommon orientation relative to antenna 1120.

FIG. 16 represents a composite retransmission response of label 2030.This composite response is determined by the combination of circuits2111, 2100, and 2101. Label 2030 represents a number having 4 bitpositions, each position corresponding to a respective frequency. Label2030 represent the number 0011 because label 2030 has circuitscorresponding to bit positions 0 and 1, and has no circuitscorresponding to bit positions 2 and 3.

FIG. 17A shows a curve REF₋₋ C of signal intensity (average power) online 1171 at the output of demodulator 1127, versus signal intensity(average power) transmitted by transmitter 1118, when band pass filter1128 is set to a band having a center frequency at Rref, and sine wavegenerator 1116 is set to a frequency Tref. The curve REF₋₋ C shown inFIG. 17A is a result of the added intensities of the signalsretransmitted by circuit 2111 in label 2020, circuit 2111 in label 2025,and circuit 2111 in label 2030. The dotted curve REF₋₋ 2020, having aslope of 0.71 between intensities I1 and I2 and a slope of 0 forintensities greater than I2, represents the contribution of circuit 2111in label 2020. The dotted curve REF₋₋ 2025, having a slope of 0.27between intensities I1 and I3 and a slope of 0 for intensities greaterthan I3, represents the contribution of circuit 2111 in label 2025. Thedotted curve REF₋₋ 2030, having a slope of 0.14 between intensities I1and I4 and a slope of 0 for intensities greater than I4, represents thecontribution of circuit 2111 in label 2030.

In FIG. 17A, between transmitted intensities I1 and I2, REF₋₋ C has aslope of approximately 1.13 until the intensity reaches a breakpoint atI2, at which point the response of circuit 2111 in label 2020 flattens.The respective voltage limiter 3100 in circuit 2111 in label 2020 causesthis flattened response at I2 by limiting the voltage in the respectivetransmitter 3043 in circuit 2111, and by detuning transmitter 3043 asthe increasing voltage in transmitter 3043 changes the capacitance ofthe PN junctions in voltage limiter 3100. Between transmittedintensities I2 and I3, REF₋₋ C has a slope of 0.42 until the intensityreaches a breakpoint at I3, at which point the response of circuit 2111in label 2025 flattens. The respective voltage limiter 3100 in circuit2111 in label 2025 causes this flattened response at I3 by limiting thevoltage in the respective transmitter 3043 in circuit 2111, and bydetuning transmitter 3043 as the increasing voltage in transmitter 3043changes the capacitance of the PN junctions in voltage limiter 3100.Between transmitted intensities I3 and I4, REF₋₋ C has a slope of 0.14until the intensity reaches a breakpoint at I4, at which point theresponse of circuit 2111 in label 2030 flattens. The respective voltagelimiter 3100 in circuit 2111 in label 2030 causes this flattenedresponse at I4 by limiting the voltage in the respective transmitter3043 in circuit 2111, and by detuning transmitter 3043 as the increasingvoltage in transmitter 3043 changes the capacitance of the PN junctionsin voltage limiter 3100.

Each of labels 2020, 2025, and 2030 may be conceptualized as arespective a circuit (each composed of multiple resonant circuits) forreceiving a first signal and transmitting a respective second signal,the second signal being a function of the first signal, the functionhaving a nonlinearity at a respective first transmission amplitudecorresponding to a first frequency (Tref) of the first signal. For label2020, the first transmission amplitude is I2. For label 2025, the firsttransmission amplitude is I3. For label 2030, the first transmissionamplitude is I4. (See FIG. 17A).

FIG. 17B shows a curve BIT0₁₃ C of signal intensity on line 1171 at theoutput of demodulator 1127, versus signal intensity transmitted bytransmitter 1118, when band pass filter 1128 is set to a band having acenter frequency at Rbit0, and sine wave generator 1116 is set to afrequency Tbit0. The curve BIT0₁₃ C shown in FIG. 17B is a result of theadded intensities of the signals retransmitted by circuit 2100 in label2020, and circuit 2100 in label 2030. The dotted curve BIT0--.sub. 2020,having a slope of approximately 0.71 between intensities I1 and I2 and aslope of 0 for intensities greater than I2, represents the contributionof circuit 2100 in label 2020. The dotted curve BIT0₁₃ 2030, having aslope of approximately 0.14 between intensities I1 and I4 and a slope of0 for intensities greater than I4, represents the contribution ofcircuit 2100 in label 2030.

In FIG. 17B, between transmitted intensities I1 and I2, BIT0₁₃ C has aslope of approximately 0.85 until the intensity reaches a breakpoint atI2, at which point the response of circuit 2100 in label 2020 flattens.The respective voltage limiter 3100 in circuit 2100 in label 2020 causesthis flattened response at I2 by limiting the voltage in the respectivetransmitter 3043 in circuit 2100, and by detuning transmitter 3043 asthe increasing voltage in transmitter 3043 changes the capacitance ofthe PN junctions in voltage limiter 3100. Between transmittedintensities I2 and I4, BIT0₁₃ C has a slope of approximately 0.14 untilthe intensity reaches a breakpoint at I4, at which point the response ofcircuit 2100 in label 2030 flattens. The respective voltage limiter 3100in circuit 2100 in label 2030 causes this flattened response at I4 bylimiting the voltage in the respective transmitter 3043 in circuit 2100,and by detuning transmitter 3043 as the increasing voltage intransmitter 3043 changes the capacitance of the PN junctions in voltagelimiter 3100.

FIG. 17C shows a curve BIT1₁₃ C of signal intensity on line 1171 at theoutput of demodulator 1127, versus signal intensity transmitted bytransmitter 1118, when band pass filter 1128 is set to a band having acenter frequency at Rbit1, and sine wave generator 1116 is set to afrequency Tbit1 . The curve BIT--.sub. C shown in FIG. 17C is a resultof the added intensities of the signals retransmitted by circuit 2101 inlabel 2020, circuit 2101 in label 2025, and circuit 2101 in label 2030.The dotted curve BIT1--.sub. 2020, having a slope of approximately 0.71between intensities I1 and I2 and a slope of 0 for intensities greaterthan I2, represents the contribution of circuit 2111 in label 2020. Thedotted curve BIT1₁₃ 2025, having a slope of approximately 0.27 betweenintensities I1 and I3 and a slope of 0 for intensities greater than I3,represents the contribution of circuit 2111 in label 2025. The dottedcurve BIT1₁₃ 2030, having a slope of approximately 0.14 betweenintensities I1 and I4 and a slope of 0 for intensities greater than I4,represents the contribution of circuit 2111 in label 2030.

In FIG. 17C, between transmitted intensities I1 and I2, BIT1₁₃ C has aslope of approximately 1.13 until the intensity reaches a breakpoint at1I2, at which point the response of circuit 2101 in label 2020 flattens.The respective voltage limiter 3100 in circuit 2101 in label 2020 causesthis flattened response at I2 by limiting the voltage in the respectivetransmitter 3043 in circuit 2101, and by detuning transmitter 3043 asthe increasing voltage in transmitter 3043 changes the capacitance ofthe PN junctions in voltage limiter 3100. Between transmittedintensities I2 and I3, BIT1₁₃ C has a slope of approximately 0.42 untilthe intensity reaches a breakpoint at I3, at which point the response ofcircuit 2101 in label 2025 flattens. The respective voltage limiter 3100in circuit 2101 in label 2025 causes this flattened response at I3 bylimiting the voltage in the respective transmitter 3043 in circuit 2101,and by detuning transmitter 3043 as the increasing voltage intransmitter 3043 changes the capacitance of the PN junctions in voltagelimiter 3100. Between transmitted intensities I3 and I4, BIT1₁₃ C has aslope of approximately 0.14 until the intensity reaches a breakpoint atI4, at which point the response of circuit 2101 in label 2030 flattens.The respective voltage limiter 3100 in circuit 2101 in label 2030 causesthis flattened response at I4 by limiting the voltage in the respectivetransmitter 3043 in circuit 2101, and by detuning transmitter 3043 asthe increasing voltage in transmitter 3043 changes the capacitance ofthe PN junctions in voltage limiter 3100.

FIG. 17D shows a curve BIT2--.sub. C of signal intensity on line 1171 atthe output of modulator 1127, versus signal intensity transmitted bytransmitter 1118, when band pass filter 1128 is set to a band having acenter frequency at Rbit2, and sine wave generator 1116 is set to afrequency Tbit2. The curve BIT2--.sub. C shown in FIG. 17D has nobreakpoints because none of labels 2020, 2025, and 2030 has a circuitcorresponding to bit 2.

FIG. 17E shows a curve BIT3--.sub. C of signal intensity on line 1171 atthe output of demodulator 1127, versus signal intensity transmitted bytransmitter 1118, when band pass filter 1128 is set to a band having acenter frequency at Rbit3, and sine wave generator 1116 is set to afrequency Tbit3. The curve BIT3--.sub. C shown in FIG. 17E is a resultof the added intensities of the signals retransmitted by circuit 2103 inlabel 2020, and circuit 2103 in label 2025. The dotted curve BIT3--.sub.2020, having a slope of approximately 0.71 between intensities I1 and I2and a slope of approximately 0 for intensities greater than I2,represents the contribution of circuit 2103 in label 2020. The dottedcurve BIT3--.sub. 2025, having a slope of approximately 0.27 betweenintensities I1 and I3 and a slope of 0 for intensities greater than I3,represents the contribution of circuit 2103 in label 2025.

In FIG. 17E, between transmitted intensities I1 and I2, BIT3--.sub. Chas a slope of approximately 0.98 until the intensity reaches abreakpoint at I2, at which point the response of circuit 2103 in label2020 flattens. The respective voltage limiter 3100 in circuit 2103 inlabel 2020 causes this flattened response at I2 by limiting the voltagein the respective transmitter 3043 in circuit 2103, and by detuningtransmitter 3043 as the increasing voltage in transmitter 3043 changesthe capacitance of the PN junctions in voltage limiter 3100. Betweentransmitted intensities I2 and I3, BIT3--.sub. C has a slope ofapproximately 0.27 until the intensity reaches a breakpoint at I3, atwhich point the response of circuit 2103 in label 2025 flattens. Therespective voltage limiter 3100 in circuit 2103 in label 2025 causesthis flattened response at I3 by limiting the voltage in the respectivetransmitter 3043 in circuit 2103, and by detuning transmitter 3043 asthe increasing voltage in transmitter 3043 changes the capacitance ofthe PN junctions in voltage limiter 3100.

FIG. 18 shows a procedure, performed by processor 1200 and program 1216,for reading labels 2020, 2025, and 2030. This procedure exploits thefact that the circuits within a particular label will have breakpointsin proximity to each other, because each circuit within a particularlabel will have a substantially common combination of distance relativeto transmitting antenna 1110 and orientation relative to antenna 1110.Because the circuits within a label have a substantially commonbreakpoint, the label may be conceptualized as having this commonbreakpoint. In other words, this procedure exploits the fact that eachlabel will usually exhibit a unique breakpoint, because each label willhave a unique combination of distance relative to transmitting antenna1110 and orientation relative to transmitting antenna 1110.

First, processor 1200 detects each breakpoint at the reference frequencyand allocates a record for each breakpoint (step 18010). (In thepreferred embodiments, when a label is present, there will always be abreakpoint at the reference frequency, because each label has areference circuit.) Each record includes a REFERENCE--.sub. BREAKPOINTfield for recording the transmission intensity on antenna 1110 at whichthe breakpoint occurred, and a LABEL--.sub. VALUE field for storing alabel value corresponding to the breakpoint stored in theREFERENCE--.sub. BREAKPOINT field.

Next, Processor 1200 sets the system to detect the least significantbit, by setting sine wave generator 1116 to the frequency Tbit0 andsetting band pass filter 1128 to a band centered around Rbit0. (step18020). In step 18020, processor 1200 also performs the variableassignment BIT--.sub. POSITION =0.

In step 18030, processor 1200 searches for each breakpoint at thepresently selected frequency. For each such breakpoint, processor 1200sets a bit in the LABEL--.sub. VALUE field of the record having aREFERENCE--.sub. BREAKPOINT field value in proximity to the breakpoint(step1 8030).

Steps 18070 and 18080 repeat step 18030 for the remaining bit positions.In step 18080, processor 1200 sets sine wave generator 1116 totransmission frequency corresponding to a bit position; sets band passfilter 1128 to a center frequency corresponding to the bit position; andperforms the variable assignment BIT--.sub. POSITION =BIT--.sub.POSITION +1.

FIG. 19 shows the procedure of step 18010 shown in FIG. 18 in moredetail. The procedure shown in FIG. 19 is similar to that shown in FIG.11, except that the procedure of FIG. 19 finds multiple breakpoints.Instead of terminating after a breakpoint is found, as is done in theprocedure of FIG. 11, the procedure of FIG. 19 records the existence ofa found breakpoint and then continues to increment the transmissionintensity to search for additional breakpoints. Processor 1200 collectsdata along a response curve, such as the curve REF₋₋ C shown in FIG.17A. To determine where the breakpoints exist, processor 1200 processesthe curve by segments, and detects whether the percentage change inslope between two segments is greater than 20%. More specifically,processor 1200 sets an initial intensity (I1) for amplifier 1118 andsets a variable RECORD--.sub. COUNT =0. (step 19010).

In step 19020, processor 1200 detects a reference segment. Processor1200 causes amplifier 1118 to transmit the present intensity,TRANSMITTED_(T), on antenna 1110; detects a received intensity,RECEIVED_(T), (through antenna 1120, filter 1128, demodulator 1127, andA/D converter 1126) by squaring the value on signal line 1173 at theoutput of A/D converter 1126; performs the variable assignmentsTRANSMITTED₁ =TRANSMITTED_(T), RECEIVED₁ =RECEIVED_(T) ; incrementsTRANSMITTED_(T) ; causes amplifier 1118 to transmit at TRANSMITTED_(T) ;detects a received intensity, RECEIVED_(T), by squaring the value onsignal line 1173; and performs the variable assignments TRANSMITTED₂=RETRANSMITTED_(T), RECEIVED₂ =RECEIVED_(T).

Processor 1200 then determines whether the transmission intensity limitIMAX has been reached, whether TRANSMITTED_(T) >IMAX (step 19025). Ifthe limit has been reached, the procedure of FIG. 19 ends. Otherwise,processor 1200 detects the next segment by performing the variableassignments TRANSMITTED_(T-1) =TRANSMITTED_(T),RECEIVED_(T-1)=RECIEIVED_(T) ; incrementing TRANSMITTED_(T) by a constant IDELTA ;causing amplifier 1118 to transmit at TRANSMITTED_(T) ; and detecting areceived intensity RECEIVED_(T). (step 19030), whereinIDELTA=(IMAX-I1)200.

Processor 1200 then processes a difference between the slope of thecurrent segment (SLOPE_(T)) and the slope of the reference segment(SLOPE_(p)), by comparing the absolute value of the expression ##EQU4##to 0.2 (step 19035), wherein ##EQU5## and ##EQU6## If this expression isgreater than 0.2, a breakpoint exists and processor 1200 executes step19045, which performs the variable assignments RECORD₋₋ COUNT=RECORD₋₋COUNT+1; R₋₋ ARRAY [RECORD₋₋ COUNT, REFERENCE₋₋BREAKPOINT]=TRANSMITTED_(T) ; R₋₋ ARRAY [RECORD₋₋ COUNT, LABEL₋₋VALUE]=0; TRANSMITTED_(T) =TRANSMITTED_(T) +5*IDELTA,

wherein R₋₋ ARRAY is an array of records.

Step 19045 increments TRANSMITTED_(T) by five times IDELTA so that thenext part of the curve to be processed will be removed from thebreakpoint recorded in the current invocation of step 19045, therebyensuring that multiple records are not allocated for a singlebreakpoint. In other words, ensuring that the next curve part is not tooclose to the currently recorded breakpoint is a safeguard in case thebreakpoint is spread out over a curve part of greater than IDELTA.

FIG. 20 shows a processing of step 18030 of FIG. 18 in more detail. Theprocedure of FIG. 20 is similar to that of FIG. 19, except that when abreakpoint is processor 1200 searches through each record in R₋₋ ARRAYand sets a bit in the LABEL₋₋ VALUE field of the record having aREFERENCE₋₋ BREAKPOINT near the current breakpoint. Processor 1200collects data along a response curve, such as the curve BIT0₁₃ C shownin FIG. 17B. To determine where the breakpoints exist, processor 1200processes the curve by segments, and detects whether the percentagechange in slope between two segments is greater than 20%. Morespecifically, processor 1200 sets an initial intensity (I1) foramplifier 1118. (step 20010).

In step 20020, processor 1200 detects a reference segment. Processor1200 causes amplifier 1118 to transmit the present intensity,TRANSMITTEDT, on antenna 1110; detects a received intensity,RECEIVED_(T), (through antenna 1120, filter 1128, demodulator 1127, andA/D converter 1126) by squaring the value on signal line 1173 at theoutput of A/D converter 1126; performs the variable assignmentsTRANSMITTED₁ =TRANSMITTED_(T), RECEIVED₁ =RECEIVED_(T) ; incrementsTRANSMITTED_(T) ; causes amplifier 1118 to transmit at TRANSMITTED_(T) ;detects a received intensity, RECEIVED_(T), by squaring the value onsignal line 1173; and performs the variable assignments TRANSMITTED₂=TRANSMITTED_(T), RECEIVED₂ =RECEIVED_(T).

Processor 1200 then determines whether the transmission intensity limithas been reached (step 20025). If the limit has been reached, theprocedure of FIG. 20 ends. Otherwise, processor 1200 detects the nextsegment by performing the variable assignments TRANSMITTED_(T-1)=TRANSMITTED_(T), RECEIVED_(T-1) =RECIEIVED_(T) ; incrementingTRANSMITTED_(T) ; causing amplifier 1118 to transmit at TRANSMITTED_(T); and detecting a received intensity RECEIVED_(T). (step 20030).Processor 1200 then processes a difference between the slope of thecurrent segment (SLOPE_(T)) and the slope of the reference segment(SLOPE_(p)), by comparing the absolute value of the expression ##EQU7##to 0.2 (step 20035), wherein ##EQU8## and ##EQU9## If this expression isgreater than 0.2, a breakpoint exists and processor 1200 executes step20045, which includes the following FOR loop, show in TABLE 1:

                  TABLE 1                                                         ______________________________________                                        FOR I=1 TO RECORD.sub.-- COUNT                                                IF ABS (R.sub.-- ARRAY [I, REFERENCE.sub.-- BREAKPOINT]-                      TRANSMITTED.sub.T) <                                                          INTRA.sub.-- LABEL.sub.-- VARIANCE                                            THEN                                                                          R.sub.-- ARRAY [I, LABEL.sub.-- VALUE] =                                      R.sub.-- ARRAY [I, LABEL.sub.-- VALUE] OR (1 BIT.sub.-- POSITION);            ______________________________________                                    

wherein I is a variable used to index to a particular record, INTRA₋₋LABEL₋₋ VARIANCE is a constant having a value reflecting differencesbetween the resonant circuits of a given label, OR is a bit wise logicalOR operator, and " " is a shift operator: 1 0=1 (0001 binary), 1 1=2(0010 binary), 1 2=4 (0100 binary), 1 3=8 (1000 binary), etc. Afterexecuting this FOR LOOP, step 20045 performs the following variableassignment TRANSMITTED_(T) =TRANSMITTED_(T) 5*IDELTA.

Processing of the second preferred method to read labels 2020, 2025, and2030 shown in FIG. 12, will now be described in more detail. In theprogram fragments shown in the description below, text appearing afterand exclamation points ("!") denotes comments for documenting a programstatement. These comments are for the benefit of a person reading theprogram and are not executed by processor 1200.

Detection of Breakpoint for Each Label

Processor 1200 determines each breakpoint at the reference frequency bysetting sine wave generator 116 to the frequency to Tref and settingband pass filter 1128 to a band centered around Rref and executing theprocedure outlined in FIG. 19 (step 18010). In FIG. 19, after executingstep 19010, the first execution of step 19020 detects a referencesegment between I1 and I1+IDELTA. In accordance with the previousdescription of FIG. 17A, the value of SLOPE_(p) is 1.13. Subsequently,processor 1200 repeatedly executes steps 19025, 19030, and 19035 untilstep 19030 detects a breakpoint segment, having a slope at least 20%different than SLOPE_(p). In other words, processor 1200 repeatedlyexecutes steps 19025, 19030, and 19035 the SLOPE_(T) will be less than0.904. (Although the slope of the curve REF₋₋ C is 0.42 between I2 andI3, the breakpoint segment detected in step 19030 may straddle intensityI2, resulting in a value of SLOPE_(T) of between 1.13 and 0.42.) Step19045 then executes the instructions:

    ______________________________________                                        ! Set RECORD.sub.-- COUNT to 1.                                               RECORD.sub.-- COUNT = RECORD.sub.-- COUNT + 1;                                !                                                                             ! Record the first reference breakpoint.                                       This will be a number in proximity to I2.                                    !                                                                             R.sub.-- ARRAY [RECORD.sub.-- COUNT,                                           REFERENCE.sub.-- BREAKPOINT]=TRANSMITTED.sub.T ;                             !                                                                             ! Clear label value field for subquent detection of                             the label value for this first reference                                    ! breakpoint                                                                  !                                                                             R.sub.-- ARRAY [RECORD.sub.-- COUNT,                                          !                                                                              LABEL.sub.-- VALUE]=0;                                                       ! Set next intensity away from presently-recorded breakpoint,                  to prevent unwanted                                                          ! redetection of the breakpoint.                                              !                                                                             TRANSMITTED.sub.T =TRANSMITTED.sub.T +5 * IDELTA                              ______________________________________                                    

Step 19020 then detects a new reference segment, having a slope of 0.42.Processor 1200 then repeatedly executes steps 19025, 19030, and 19035until step 19030 detects a segment, having a slope of 0.336 or less(0.336 being 20% different from 0.42).

Step 19045 then executes the instructions:

    ______________________________________                                        ! Set RECORD.sub.-- COUNT to 2.                                               !                                                                             ! RECORD.sub.-- COUNT = RECORD.sub.-- COUNT + 1;                              ! Record the second reference breakpoint.                                      This will be a number in proximity to I3.                                    !                                                                             ! R.sub.-- ARRAY [RECORD.sub.-- COUNT,                                          REFERENCE.sub.-- BREAKPOINT]=TRANSMITTED.sub.T ;                            !                                                                             ! Clear label value field for subquent detection of                            the label value for second reference                                         ! breakpoint                                                                  !                                                                             R.sub.-- ARRAY [RECORD.sub.-- COUNT,                                           LABEL.sub.-- VALUE]=0;                                                       ! Set next intensity away from                                                  presently-recorded breakpoint, to prevent unwanted                          ! redetection of the breakpoint.                                              !                                                                             TRANSMITTED.sub.T =TRANSMITTED.sub.T +5 * IDELTA                              ______________________________________                                    

Step 19020 then detects a new reference segment, having a slope of 0.14.Processor 1200 then repeatedly executes steps 19025, 19030, and 19035until step 19030 detects a segment, having a slope of 0.112 or less(0.112 being 20% different than 0.14).

Step 19045 then executes the instructions:

    ______________________________________                                        ! Set RECORD.sub.-- COUNT to 3.                                               !                                                                             RECORD.sub.-- COUNT = RECORD.sub.-- COUNT + 1;                                !                                                                             ! Record the third reference breakpoint.                                        This will be a number in proximity to I4.                                   !                                                                             R.sub.-- ARRAY [RECORD.sub.-- COUNT,                                           REFERENCE.sub.-- BREAKPOINT]=TRANSMITTED.sub.T ;                             !                                                                             ! Clear label value field for subquent detection of                            the label value for this third reference                                     ! breakpoint                                                                  !                                                                             R.sub.-- ARRAY [RECORD.sub.-- COUNT,                                           LABEL.sub.-- VALUE]=0;                                                       !                                                                             ! Set next intensity away from presently-recorded                              breakpoint, to prevent unwanted                                              ! redetection of the breakpoint.                                              !                                                                             TRANSMITTED.sub.T =TRANSMITTED.sub.T +5 * IDELTA                              ______________________________________                                    

Step 19020 then detects a reference segment, having a slope of 0.Processor 1200 then repeatedly executes 19025, 19030, and 19035 untilTRANSMITTED_(T) is greater than IMAX, at which point the procedure ofFIG. 19 terminates.

Thus, after execution of step 18010, processor 1200 has allocated threerecords, each with a LABEL₋₋ VALUE field of 0. The FOR loop shown inTABLE 1 above is a loop FROM 1 TO 3, because RECORD₋₋ COUNT is equal to3.

Detection of Bit0 for Each Label

Processor 1200 then sets the system to detect the least significant bit,by setting sine wave generator 1116 to the frequency Tbit0 and settingband pass filter 1128 to a band centered around Rbit0; and performs thevariable assignment BIT₋₋ POSITION=0. (step 18020). Processor 1200 thenprocesses each breakpoint at the transmitted frequency Tbit0, (step18030), by executing the procedure outlined in FIG. 20. In other words,processor 1200 collects data along the response curve bit0₋₋ C shown inFIG. 17B.

After setting an initial intensity of I1 for amplifier 1118 (step20010), processor 1200 detects a reference segment between I1 and I1+IDELTA, having a slope of 0.85. Processor 1200 then repeatedly executessteps 20025, 20030, and 20035 until step 20030 detects a breakpointsegment, having a slope of 0.68 or less (0.68 being 20% less than 0.85).Now, TRANSMITTED_(T) has a value in the vicinity of I2. Thus, whenprocessor 1200 executes the FOR loop of step 20045, shown in TABLE 1above, the IF statement condition will be true the first time throughthe loop, because R₋₋ ARRAY [1, REFERENCE₋₋ BREAKPOINT] also has a valuein the vicinity of I2. More specifically, the following expression willbe true:

    ABS (R.sub.-- ARRAY [1, REFERENCE.sub.-- BREAKPOINT]-TRANSMITTED.sub.T)<INTRA.sub.-- LABEL.sub.-- VARIANCE.

Thus, the first time through the loop processor 1200 executes the THENclause of TABLE 1:

    R.sub.-- ARRAY [1, LABEL.sub.-- VALUE]=0000 or 1 0.

Thus, immediately after this invocation of step 20045, R₋₋ ARRAY [1,LABEL₋₋ VALUE] is equal to 0001, R₋₋ ARRAY [2, LABEL₋₋ VALUE] is equalto 0000, and R₋₋ ARRAY[3, LABEL₋₋ VALUE] is equal to 0000.

Subsequently, step 20045 detects a reference segment between I2 and I4,having a slope of 0.14. Subsequently, processor 1200 executes steps20025, 20030, and 20035, until step 20030 detects a segment extendingpast intensity I4, having a slope of 0.112 or less (0.112 being 20% lessthan 0.14). Now, TRANSMITTED_(T) has a value in the vicinity of I4.Thus, when processor 1200 executes the FOR loop of step 20045, shown inTABLE 1 above, the IF statement condition will be true the third timethrough the loop, because R₋₋ ARRAY [3, REFERENCE₋₋ BREAKPOINT] also hasa value in the vicinity of I4. More specifically, the followingexpression will be true:

    ABS (R.sub.-- ARRAY [3, REFERENCE.sub.-- BREAKPOINT]-TRANSMITTED.sub.T)INTRA.sub.-- LABEL.sub.-- VARIANCE.

Thus, the third time through the loop processor 1200 executes the THENclause of TABLE 1:

    R.sub.-- ARRAY [3, LABEL.sub.-- VALUE]=0000 or 1 0.

Thus, immediately after this invocation of step 20045, R₋₋ ARRAY [1,LABEL₋₋ VALUE] is equal to 0001, R₋₋ ARRAY [2, LABEL₋₋ VALUE] is equalto 0000, and R₋₋ ARRAY[3, LABEL₋₋ VALUE] is equal to 0001.

Subsequently, step 20020 detects a reference segment past intensity I4having a slope 0. Processor 1200 then repeatedly executes steps 20025,20030, and 20035 until TRANSMITTED_(T) >IMAX, and the procedure of FIG.20 then terminates.

Detection of Bit 1 for Each Label

Processor 1200 then sets the system to detect the next most significantbit, by setting sine wave generator 1116 to the frequency Tbit1 andsetting band pass filter 1128 to a band centered around Rbit1; andperforms the variable assignment BIT₋₋ POSITION=1. (step 18080).Processor 1200 then processes each breakpoint at the transmittedfrequency Tbit1, (step 18030), by executing the procedure outlined inFIG. 20. In other words, processor 1200 collects data along the responsecurve bit1₋₋ C shown in FIG. 17C.

After setting an initial intensity of I1 for amplifier 1118 (step20010), processor 1200 detects a reference segment between I1 andI1+IDELTA, having a slope of 1.13. Processor 1200 then repeatedlyexecutes steps 20025, 20030, and 20035 until step 20030 detects abreakpoint segment, having a slope of 0.90 or less (0.90 being 20% lessthan 1.13). Now, TRANSMITTED_(T) has a value in the vicinity of I2.Thus, when processor 1200 executes the FOR loop of step 20045, shown inTABLE 1 above, the IF statement condition will be true the first timethrough the loop, because R₋₋ ARRAY [1, REFERENCE₋₋ BREAKPOINT] also hasa value in the vicinity of I2. More specifically, the followingexpression will be true:

    ABS (R.sub.-- ARRAY [1, REFERENCE.sub.-- BREAKPOINT]-TRANSMITTED.sub.T)INTRA.sub.-- LABEL.sub.-- VARIANCE.

Thus, the first time through the loop processor 1200 executes the THENclause of TABLE 1:

    R.sub.-- ARRAY [1, LABLE.sub.-- VALUE]=0001 or 1 1.

Thus, immediately after this invocation of step 20045, R₋₋ ARRAY [1,LABLE₋₋ VALUE] is equal to 0011, R₋₋ ARRAY [2, LABLE₋₋ VALUE] is equalto 0000, and R₋₋ ARRAY [3, LABLE₋₋ VALUE] is equal to 0001.

Subsequently, step 20045 detects a reference segment between I2 and I3,having a slope of 0.42. Subsequently, processor 1200 executes steps20025, 20030, and 20035, until step 20030 detects a segment, having aslope of 0.34 or less (0.34 being 20% less than 0.42). Now,TRANSMITTED_(T) has a value in the vicinity of I3. Thus, when processor1200 executes the FOR loop of step 20045, shown in TABLE 1 above, the IFstatement condition will be true the second time through the loop,because R₋₋ ARRAY [2, REFERENCE₋₋ BREAKPOINT] also has a value in thevicinity of I3. More specifically, the following expression will betrue:

    ABS (R.sub.-- ARRAY [2, REFERENCE.sub.-- BREAKPOINT]-TRANSMITTED.sub.T)INTRA.sub.-- LABEL.sub.-- VARIANCE.

Thus, the second time through the loop processor 1200 executes the THENclause of TABLE 1:

    R.sub.-- ARRAY [2, LABLE.sub.-- VALUE]=0000 or 1 1.

Thus, immediately after this invocation of step 20045, R₋₋ ARRAY [1,LABLE₋₋ VALUE] is equal to 0011, R₋₋ ARRAY [2, LABLE₋₋ VALUE] is equalto 0010, and R₋₋ ARRAY [3, LABLE₋₋ VALUE] is equal to 0001.

Subsequently, step 20045 detects a reference segment between I3 and I4,having a slope of 0.14. Subsequently, processor 1200 executes steps20025, 20030, and 20035, until step 20030 detects a segment, having aslope of 0.11 or less (0.11 being 20% less than 0.14). Now,TRANSMITTED_(T) has a value in the vicinity of I4. Thus, when processor1200 executes the FOR loop of step 20045, shown in TABLE 1 above, the IFstatement condition will be true the third time through the loop,because R₋₋ ARRAY [3, REFERENCE₋₋ BREAKPOINT] also has a value in thevicinity of I4. More specifically, the following expression will betrue:

    ABS (R.sub.-- ARRAY [3, REFERENCE.sub.-- BREAKPOINT]-TRANSMITTED.sub.T)INTRA.sub.-- LABEL.sub.-- VARIANCE.

Thus, the third time through the loop processor 1200 executes the THENclause of TABLE 1:

    R.sub.-- ARRAY [3, LABLE.sub.-- VALUE]=0001 or 1 1.

Thus, immediately after this invocation of step 20045, R₋₋ ARRAY [1,LABLE₋₋ VALUE] is equal to 0011, R₋₋ ARRAY [2, LABLE₋₋ VALUE] is equalto 0010, and R₋₋ ARRAY [3, LABLE₋₋ VALUE] is equal to 0011.

Subsequently, step 20020 detects a reference segment past intensity I4having a slope 0. Processor 1200 then repeatedly executes steps 20025,20030, and 20035 until TRANSMITTED_(T) >IMAX, and the procedure of FIG.20 then terminates.

Detection of Bit 2 for Each Label

Processor 1200 then sets the system to detect the next most significantbit, by setting sine wave generator 1116 to the frequency Tbit2 andsetting band pass filter 1128 to a band centered around Rbit2; andperforms the variable assignment BIT₋₋ POSITION=2. (step 18080).Processor 1200 then attempts to process each breakpoint at thetransmitted frequency Tbit2, (step 18030), by executing the procedureoutlined in FIG. 20. In other words, processor 1200 collects data alongthe response curve bit2₋₋ C shown in FIG. 17D.

After setting an initial intensity of I1 for amplifier 1118 (step20010), processor 1200 detects a reference segment between I1 and I1+IDELTA, having a slope of 0. Processor 1200 then repeatedly executessteps 20025, 20030, and 20035 until TRANSMITTED_(T) >IMAX, and theprocedure of FIG. 20 then terminates.

Detection of Bit 3 for Each Label

Processor 1200 then sets the system to detect the next most significantbit, by setting sine wave generator 1116 to the frequency Tbit3 andsetting band pass filter 1128 to a band centered around Rbit3; andperforms the variable assignment BIT₋₋ POSITION=3. (step 18080).Processor 1200 then processes each breakpoint at the transmittedfrequency Tbit3, (step 18030), by executing the procedure outlined inFIG. 20. In other words, processor 1200 collects data along the responsecurve bit3₋₋ C shown in FIG. 17E.

After setting an initial intensity of I1 for amplifier 1118 (step20010), processor 1200 detects a reference segment between I1 and I1+IDELTA, having a slope of 0.98. Processor 1200 then repeatedly executessteps 20025, 20030, and 20035 until step 20030 detects a breakpointsegment, having a slope of 0.78 or less (0.78 being 20% less than 0.98).Now, TRANSMITTED_(T) has a value in the vicinity of I2. Thus, whenprocessor 1200 executes the FOR loop of step 20045, shown in TABLE 1above, the IF statement condition will be true the first time throughthe loop, because R₋₋ ARRAY [1, REFERENCE₋₋ BREAKPOINT] also has a valuein the vicinity of I2. More specifically, the following expression willbe true:

    ABS (R.sub.-- ARRAY [1, REFERENCE.sub.-- BREAKPOINT]-TRANSMITTED.sub.T)INTRA.sub.-- LABEL.sub.-- VARIANCE.

Thus, the first time through the loop processor 1200 executes the THENclause of TABLE 1:

    R.sub.-- ARRAY [1, LABLE.sub.-- VALUE]=0011 or 1 3.

Thus, immediately after this invocation of step 20045, R₋₋ ARRAY [1,LABLE₋₋ VALUE] is equal to 1011, R₋₋ ARRAY [2, LABLE₋₋ VALUE] is equalto 0010, and R₋₋ ARRAY [3, LABLE₋₋ VALUE] is equal to 0011.

Subsequently, step 20045 detects a reference segment between I2 and I3,having a slope of 0.27. Subsequently, processor 1200 executes steps20025, 20030, and 20035, until step 20030 detects a segment, having aslope of 0.22 or less (0.22 being 20% less than 0.27). Now,TRANSMITTED_(T) has a value in the vicinity of I3. Thus, when processor1200 executes the FOR loop of step 20045, shown in TABLE 1 above, the IFstatement condition will be true the second time through the loop,because R₋₋ ARRAY [2, REFERENCE₋₋ BREAKPOINT] also has a value in thevicinity of I2. More specifically, the following expression will betrue:

    ABS (R.sub.-- ARRAY [2, REFERENCE.sub.-- BREAKPOINT]-TRANSMITTED.sub.T)INTRA.sub.-- LABEL.sub.-- VARIANCE.

Thus, the second time through the loop processor 1200 executes the THENclause of TABLE 1:

    R.sub.-- ARRAY [2, LABLE.sub.-- VALUE]=0010 or 1 3.

Thus, immediately after this invocation of step 20045, R₋₋ ARRAY [1,LABLE₋₋ VALUE] is equal to 1011, R₋₋ ARRAY [2, LABLE₋₋ VALUE] is equalto 1010, and R₋₋ ARRAY [3, LABLE₋₋ VALUE] is equal to 0011.

Subsequently, step 20045 detects a reference segment after I3, having aslope of 0. Subsequently, processor 1200 executes steps 20025, 20030,and 20035, until TRANSMITTED_(T) >IMAX, and the procedure of FIG. 20then terminates.

Thus, for each of labels 2020, 2025, and 2030, processor 1200 determinesa respective set of other frequencies of the first signal at which therespective second signal has a nonlinearity corresponding to therespective first transmission amplitude. For label 2025, the set ofother frequencies is Tbit1 and Tbit3. For label 2030, the set of otherfrequencies is Tbit0 and Tbit1.

Thus, the LABLE₋₋ VALUE field of each record in R₋₋ ARRAY stores anarticle identification signal for a respective suitcase.

The second preferred method allows for a substantial difference in breakdown voltage among labels, as long as the differences in break downvoltage among the circuits of any particular label does not result in adifference in breakpoints greater than INTRA₋₋ LABEL₋₋ VARIANCE. Thus,the respective labels may be made from different batches of material andneed not be calibrated to the extent that the resonant circuits on anyparticular label should be calibrated with each other.

The second preferred method detects a breakpoint by comparing the slopeof a reference segment (SLOPE_(p)) to a slope of a present segment(SLOPE_(T)). Other methods might be employed to make the breakpointdetection relatively insensitive to the non-linearities caused bynatural retransmitters (other than labels) in the environment of thesystem. For example, a method might be employed that also compares theslopes of adjacent segments.

The constants, such as IDELTA and 5* IDELTA, may be adjusted for anoptimum trade off between design goals.

If some mechanical configurations of the preferred system might allowtwo or more labels to have distances and orientations from thetransmitting antenna causing two labels to exhibit reference breakpointsvalues that are closer than INTRA₋₋ LABEL₋₋ VARIANCE, this conflictcondition can be detected in a number of ways. First, each LABLE₋₋ VALUEfield could include redundancy bits, such as a checksum or a cyclicredundancy code, allowing the processor to verify a correct LABLE₋₋VALUE. Alternatively, the processor may consult a table after reading aparticular LABLE₋₋ VALUE, the absence of the LABLE₋₋ VALUE in the tableindicating this conflict condition.

Alternatively, the processor may compare the sharpness of thebreakpoints corresponding to the various one-valued bits in a read code.Normally, similar sharpnessess indicate that the breakpoints correspondto a single label. In this conflict condition, however, the referencebreakpoint sharpness may have a relatively high value resulting frommultiple labels having the same breakpoint at the reference frequency,while a breakpoint corresponding to a particular bit position may have asubstantially lower sharpness resulting from only a single label havingthe breakpoint.

Although the preferred embodiments of the invention employ resonant tagcircuits that retransmit in response to receiving an interrogationsignal, the invention may employ other types of schemes, such asdetection of interrogator antenna loading caused by the label circuits.In such a system, detuning caused by a voltage limiter in the labelcircuit limits the loading on the interrogator circuit.

The illustrated conveyor belts move the labels at a slow speed relativeto the speed of execution of the preferred methods discussed above.Thus, although movement of the labels relative to the antennas changesthe breakpoints, during any particular execution of the preferred methodthe labels are in a fixed position relative to the antennas.

In the event the conflict condition described above occurs, thepreferred methods may be reexecuted after the conveyor belt moves thelabels to a new position relative to the antennas. At the new position,the breakpoint positions may have changed such that a conflict no longerexists.

Although the illustrated embodiments of the invention employ a dedicatedvoltage limiting circuit, in its broadest sense the invention may bepracticed without such a dedicated circuit since the preferred methodswill process breakpoints in the response of the label circuits,regardless of the origin of such breakpoints.

Conversely, the invention may be practiced with more complicateddedicated circuitry to generate the non-linear response, as shown inFIG. 21. FIG. 21 shows circuit 2111', which is a substitute for circuit2111 shown in FIG. 5. Circuit 2111' has the responses shown in FIG. 8Aand FIG. 9A. Battery 21010 supplies the power to band pass filter 21020,demodulator 21025, limitor 21030, and variable power wave form generator21035. A signal from a receiving antenna 21015 is filtered by a bandpass filter having a pass band centered around Tref. Band pass filter21020 applies a filtered output to demodulator 21025, which applies alevel to limitor 21030. Limitor 21030 has an output that is anincreasing function of its input until a certain voltage level isreached at the input, at which point the output remains at a constantmaximum value. The output of limitor 21030 controls variable power waveform generator 21035, which transmits a signal having a frequency Rref.

As another alternative, a label circuit might have a comparator anddigital logic to generate a non-linear retransmission response. Thus,the invention may be practiced with many types of label circuit having anon-linear response.

Thus, the invention permits label reading in a multi-label environment.

Additional advantages and modifications will readily occur to thoseskilled in the art and may learned from the practice of the invention.The invention in its broader aspects is therefore not limited to thespecific details, representative apparatus, and illustrative examplesshown and described. Accordingly, departures may be made from suchdetails without departing from the spirit or the scope of applicant'sgeneral inventive concept. The invention is defined in the followingclaims.

What is claimed is:
 1. In a system including a plurality of articleseach having a respective circuit for receiving an interrogation signaland transmitting a respective circuit signal, the circuit signal being afunction of the interrogation signal, and having a nonlinearity, thenonlinearity corresponding to a respective amplitude level of theinterrogation signal, a method comprising the steps of:detecting, foreach circuit, the respective amplitude level by varying an amplitudeused to transmit the interrogation signal to the plurality of articles;and registering the presence of each article, in response to theprevious step.
 2. The method of claim 1 wherein the generating stepincludes generating a respective plurality of digits for each article,each digit corresponding to a respective frequency in the respective setof frequencies.
 3. The method of claim 1 further including detecting,for each circuit, a respective set of other nonlinearities correspondingto the respective amplitude level.
 4. The method of claim 1 furtherincluding detecting, for each circuit, a respective set of othernonlinearities corresponding to the respective amplitude level, bycausing the interrogation signal to have a plurality of interrogationsignal frequencies.
 5. The method of claim 1 wherein detecting therespective amplitude level includes causing the interrogation signal tohave a first interrogation signal frequency at a first time, and themethod further includesdetecting, for each circuit, a respective set ofother nonlinearities by causing the interrogation signal to have otherinterrogation signal frequencies at a time different than the firsttime.
 6. The method of claim 1 wherein registering includes changing acount.
 7. The method of claim 1 wherein registering includes allocatinga record.
 8. In a system including a plurality of articles each having arespective circuit for receiving an interrogation signal andtransmitting a respective circuit signal, the circuit signal being afunction of the interrogation signal, and having a nonlinearity, thenonlinearity corresponding to a respective amplitude level of theinterrogation signal, a method comprising the steps of:detecting, foreach circuit, the respective amplitude level by varying an amplitudeused to transmit the interrogation signal to the plurality of articles;and recording the presence of each article, in response to the previousstep.
 9. The method of claim 8 wherein recording includes changing acount.
 10. The method of claim 8 wherein recording includes allocating arecord.
 11. The method of claim 8 further including detecting, for eachcircuit, a respective set of other nonlinearities corresponding to therespective amplitude level.
 12. The method of claim 8 further includingdetecting, for each circuit, a respective set of other nonlinearitiescorresponding to the respective amplitude level, by causing theinterrogation signal to have a plurality of interrogation signalfrequencies.
 13. The method of claim 8 wherein detecting the respectiveamplitude level includes causing the interrogation signal to have afirst interrogation signal frequency at a first time, and the methodfurther includesdetecting, for each circuit, a respective set of othernonlinearities by causing the interrogation signal to have otherinterrogation signal frequencies at a time different than the firsttime.
 14. The apparatus of claim 8 wherein the recorder includes acounter.
 15. The apparatus of claim 8 wherein the recorder includes aplurality of records.
 16. An apparatus for a system including aplurality of articles each having a respective circuit for receiving aninterrogation signal and transmitting a respective circuit signal, thecircuit signal being a function of the interrogation signal, and havinga nonlinearity, the nonlinearity corresponding to a respective amplitudelevel of the interrogation signal, the apparatus comprising:a detectorthat detects, for each circuit, the respective amplitude level byvarying an amplitude used to transmit the interrogation signal to theplurality of articles, and performing a comparison; and a recorder thatrecords the presence of each article, in response to the comparison. 17.The apparatus of claim 16 further including logic that detects, for eachcircuit, a respective set of other nonlinearities corresponding to therespective amplitude level.
 18. The apparatus of claim 16 furtherincluding logic that detects, for each circuit, a respective set ofother nonlinearities corresponding to the respective amplitude level, bycausing the interrogation signal to have a plurality of interrogationsignal frequencies.
 19. The apparatus of claim 16 wherein the detectordetects the respective amplitude level by causing the interrogationsignal to have a first interrogation signal frequency at a first time,and the apparatus further includes logic that detects, for each circuit,a respective set of other nonlinearities by causing the interrogationsignal to have other interrogation signal frequencies at a timedifferent than the first time.
 20. In a system including a plurality ofarticles each having a respective circuit for receiving an interrogationsignal and transmitting a respective circuit signal, the circuit signalbeing a function of the interrogation signal, and having a nonlinearity,the nonlinearity corresponding to a respective amplitude level of theinterrogation signal, an apparatus comprising:means for detecting, foreach circuit, the respective amplitude level by varying an amplitudeused to transmit the interrogation signal to the plurality of articles,and performing a comparison; and means for recording the presence ofeach article, in response to the comparison.
 21. The apparatus of claim20 wherein the means for recording includes a counter.
 22. The apparatusof claim 20 wherein the means for recording includes a plurality ofrecords.
 23. The apparatus of claim 20 further including logic thatdetects, for each circuit, a respective set of other nonlinearitiescorresponding to the respective amplitude level.
 24. The apparatus ofclaim 20 further including logic that detects, for each circuit, arespective set of other nonlinearities corresponding to the respectiveamplitude level, by causing the interrogation signal to have a pluralityof interrogation signal frequencies.
 25. The apparatus of claim 20wherein the means for detecting detects the respective amplitude levelby causing the interrogation signal to have a first interrogation signalfrequency at a first time, and the apparatus further includes logic thatdetects, for each circuit, a respective set of other nonlinearities bycausing the interrogation signal to have other interrogation signalfrequencies at a time different than the first time.